Photovoltaic devices, particularly photovoltaic devices manufactured from deposited layers of thin film semiconductor material, preferably include a back reflector disposed beneath the photoactive semiconductor layers. A back reflector re-directs light which has passed through the photovoltaic body unabsorbed, back through that body for further absorption. The use of a back reflector increases the efficiency of the photovoltaic device and permits the use of relatively thinner layers of photoactive material, thereby enhancing the collection efficiency of the device as well as producing a savings in materials. In many instances, the back reflectors are textured so as to provide a diffuse reflecting surface which increases the path length of the reflected light and also provides for enhanced internal reflection.
It is fundamental that a back reflector must be highly reflective of light having a wavelength corresponding to the photo response spectrum of the semiconductor layers. In general, it has been found that metals such as aluminum, silver, and copper are good reflectors for thin film photovoltaic devices, such as silicon alloy devices. It is also most important that the materials comprising the back reflector not react with one another or with other materials in the photovoltaic device, either during manufacture of the device or its use. Such reactions can degrade the efficiency of the device and even render it inoperative.
Back reflector structures must also be mechanically compatible with the remaining layers of the photovoltaic device. The thinness of the layers can make the device sensitive to a number of mechanical defects. Spikes or other protrusions in the substrate or in the back reflector can produce shunts or short circuits through the semiconductor layers, thereby compromising device function. Additionally, a back reflector structure which cracks, deforms or otherwise delaminates from the substrate can destroy superadjacent semiconductor layers to the detriment of the device function. Accordingly, it will be appreciated that back reflectors for thin film photovoltaic devices should be highly reflective of light, non-reactive with adjacent layers of the device, and mechanically compatible with device structure and processing.
The simplest prior art approach to provide a back reflector comprises depositing the various layers of the photovoltaic device upon a highly polished substrate, typically stainless steel. However, while stainless steel is mechanically stable and non-reactive, it is not very highly reflective of light; the integrated reflectivity of most stainless steels is only about 45%. Furthermore, providing a textured reflective surface to the stainless steel is somewhat difficult. As a consequence, more sophisticated back reflector structures have been developed. For example, U.S. Pat. No. 5,101,260 discloses a multi-layered, light-scattering back reflector for a photovoltaic device, said back reflector including a first relatively hard, textured layer atop a substrate, a second highly reflective layer conformally disposed atop the first layer, and a trasparent oxide layer deposited upon the highly reflective layer, the oxide layer adapted to enhance light scattering and optical coupling. U.S. Pat. No. 5,296,045 discloses a composite back reflector for a photovoltaic device including an electrically conducting, texturizing layer disposed atop a substrate and a light reflecting layer conformally disposed atop the texturizing layer, the two layers being fabricated from materials which are mutually non-reactive over a temperature range of -20 degrees C. to 450 degrees C.
Prior attempts to use silver as the highly reflective material from which to fabricate the back reflectors for photoresponsive devices have been only partly successful. Silver and silver alloys present their own particular problems when employed as highly reflective back reflector material; i.e., silver, due to its relatively soft nature, tends easily to deform, particularly during the deposition of the body of semiconductor material, thus causing a loss of any textured surface. Also, deformation of the silver can create short circuit defects in cells, thereby decreasing overall production yields. Silver is also expensive as compared to other back reflector materials, such as, for example, aluminum. Furthermore, contact between the silver reflector and overlying layers can cause the silver to tarnish, thus reducing its reflectivity. Silver ions are also known to migrate into the semiconductor body, particularly if moisture enters the device. Furthermore, shadows falling on the device can cause a reverse bias which draws silver ions into the regions of the cell. While the afore-mentioned U.S. Pat. No. 5,296,045 to some extent overcomes some of these problems, it still requires the use of expensive silver as the primary back reflector material.
Aluminum, with or without a silicon alloying agent, is highly reflective of light (integrated reflectivity of about 88%), and the texture of the layer may be controlled via the parameters of the deposition process. Aluminum is somewhat reactive and, as a consequence, in most instances, it is coated with a protective layer of a conductive material, such as a metallic oxide, which may optionally include an additional metal. In some instances, the aluminum is deposited as a specular (smooth) reflector, and a textured protective layer is deposited thereatop to provide for light scattering. Back reflectors of this type are presently incorporated in a wide variety of photovoltaic devices and are disclosed, for example, in the aforementioned U.S. Pat. No. 5,101,260.
However, previous attempts to employ aluminum as the highly reflective material of a back reflector for a photoresponsive device have also been only partially successful because of the interdiffusion problems alluded to herein above. More particularly, when the amorphous silicon alloy material is deposited upon highly reflective material fabricated from aluminum, interdiffusion of the silicon and the aluminum from the contiguous layers results. Obviously the photogenerating and photoconductive properties of the body of silicon alloy material, as well as the reflective properties of the back reflector suffer. This, combined with the fact that aluminum is inherently less reflective than silver, has resulted in a preference for silver, despite its costliness and yield problems. While aluminum is a good light reflector, silver and copper are better yet, particularly for those portions of the electromagnetic spectrum which correlate to the response spectrum of thin film silicon alloy materials. In an attempt to improve production yields and to realize the cost savings from using less reflective but cheaper aluminum, the aforementioned U.S. Pat. No. 5,101,260 has proposed to provide a conventional aluminum silicon alloy back reflector with a relatively thin layer of silver deposited thereupon. However, it has been found that, while the silver enhanced the efficiency of the device somewhat, the efficiency is not as good as that attained using a thick silver/zinc oxide reflector.
Thus, there still exists a need for a back reflector which has the superb reflective qualities of a thick layer of silver, but does not have the associated yield problems, and which utilizes a cheaper material such as aluminum. There particularly exists a need for such a back reflector which can easily be manufactured in large quantities and which has spectral properties that can be manipulated according to the characteristics of the incident radiation.